This kind of bipolar pulsed power supply is used, for example, in a sputtering apparatus for forming a predetermined thin film on the surface of a substrate to be processed. There is known one which comprises: a rectifying circuit for supplying DC power; and a MOSFET bridge circuit which is connected to positive and negative outputs and is made up of four switching elements. Each of the switching elements is appropriately operated by a control means so that, by alternately switching the polarity at a predetermined frequency, an arbitrary pulsed voltage is applied to a pair of targets which are output ends (electrodes). By thus switching the target alternately to anode electrode and cathode electrode, glow discharge is caused to be generated between the anode electrode and the cathode electrode. A plasma atmosphere is thus formed to thereby sputter each of the targets. According to this arrangement, there is an advantage in that the electric charges to be accumulated on the surface of the targets are canceled when the voltage of opposite phase is applied, thereby obtaining a stable electric discharge (see, e.g., patent document 1).
It is known that, in this kind of glow discharge, arcing (anomalous electric discharge) takes place by unknown causes. When arcing occurs, the impedance on the side of the plasma load rapidly decreases, and a sudden voltage drop takes place, with the resultant increase in current. In case the target is of metallic make, especially of aluminum and the like, when arcing of high arc current value locally takes place between targets, the particles or splashes (particles of several μm to several hundreds μm) to be generated by melting and emission of the targets will take place, thereby impairing good film forming.
In view of the above points, the bipolar power supply as described in the above-mentioned patent document 1 is provided with a detection circuit which detects an output current from a bridge circuit, and an inductor which restricts the increase in current at the time of occurrence of arcing. When the output current as detected by this detection circuit has exceeded a steady output current value, the switching element in operation is switched to thereby once cut off the output to the electrode. Once the overcurrent has calmed down so that the value becomes close to the steady output current value, the output to the electrode is resumed. In this case, when the output current varies beyond a certain range, it is taken or recognized as a pre-stage phenomenon (micro-arc) of arcing so that, by performing the process of extinguishing the arcing, the occurrence of arcing which is large in the amount of current change can be suppressed.
If the output from the DC power supply source has constant-voltage characteristics, the capacity component (capacitance) becomes dominant over the inductance components. Therefore, at the time of occurrence of arcing, as a result of decrease in the impedance on the side of the plasma load (depending on cases, the impedance becomes several ohms or less), the output and the plasma (load) are coupled together so that the capacitance will rapidly be discharged to the output side. As a result, there is a problem in that, even if an inductor of a small inductance value is provided, the increase in current cannot be suppressed efficiently and therefore that overcurrent flows in a short period of time (for several μs) which means that the rate of increase in current per unit time at the time of arcing is high.
In case the rate of increase in current per unit time is high, even in case micro-arc processing is performed by taking advantage of a relatively small state of current change amount, there are cases where large arc current flows within a time from the time of detection of arcing based on the change in current which takes place behind the voltage change to the time of shut off of the power supply to the plasma. The arc energy to be emitted becomes large (about two times of the current of the steady current value flows) and splashes and particles tend to be generated. Especially, when the arcing takes place in succession, the occurrence of splashes and particles cannot be substantially suppressed.
On the other hand, if there is provided an inductor of an inductance value which is larger by one digit than the inductance component of the plasma, the output from the DC power supply source becomes a constant-current characteristic, so that the rate of increase in current per unit time at the time of occurrence of arcing will be limited. However, when each of the switching elements of the bridge circuit is switched, there are cases where a higher voltage than the ordinary discharge voltage is generated. In other words, as a result of occurrence of inductance components in the plasma, overvoltage takes place at the time of polarity reversal in each of the targets. Once overvoltage occurs in this manner, there is a possibility of inducing arcing.    Patent Document 1: Japanese Patent No. 3639605